notify: do not mime encode multipart templates
Mulitpart templates should take care of this themselves.
See http://www.selenic.com/pipermail/mercurial/2009-July/027017.html
Also catch potential parsing errors gracefully.
% convert while testing all possible outputs
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% check tags are in UTF-8
'1c1b4062874e8197a68e7ce9e498479a37d0215c branch\xc3\xa9e\n'
'edc96568720ef79eefa6372677b18d855de3abee branch\xc3\xa9\n'